This application claims Paris Convention priority of DE 199 50 270.6 filed Oct. 18, 1999 the complete disclosure of which is hereby incorporated by reference.
The invention concerns a bone plate for osteosynthesis, comprising a plate-shaped basic body and a lid, wherein the basic body comprises at least one opening for receiving a bone screw mounting same e.g. to a vertebra, in that the opening receives the screw head of the bone screw.
Bone plates are known per se. Bone plates of this type are e.g. mounted to vertebrae to stabilize same. For this purpose, the individual bone plates are connected to one another via rods, wherein the rods are mounted, in particular clamped, to the bone plates.
The bone plates are mounted to the vertebrae by means of bone screws which penetrate the bone plates and are screwed into the vertebrae. The bone plate is generally held by the screw head.
It has thereby turned out that the bone plate initially tightly abuts the vertebra, but may loosen with time, in particular if the bone screw loosens within the vertebra or if the bone changes in the abutment area of the bone plate.
It is therefore the underlying purpose of the invention to provide a bone plate which reduces the danger of subsequent loosening.
This object is achieved in accordance with the invention in that, in a bone plate of the initially described type, a surface structure is provided on at least a partial area of the opening surface facing the screw head.
The surface structure in the opening on which the screw head abuts results in a retaining effect, i.e. in a turning-out direction of the screw head. The bone plate is thus connected, both in frictional as well as in positive engagement, to the screw head and thus to the screw. The positive engagement reduces the danger that the screw loosens, i.e. loses the fixed support in the bone. There is furthermore a connection between screw head and bone plate even if the bone changes its shape in the abutment area of the bone plate.
In a preferred manner, the opening is circular. Such openings permit easy screwing-in and abutment of the screw head.
Although only part of the periphery of the opening must be provided with a surface structure, in a preferred embodiment, the surface structure extends over the entire inner circumference of the opening. This has the essential advantage that the head of the bone screw is also fixed over its entire outer periphery by anchoring it in the opening in positive engagement.
A further development provides that the area of the opening facing away from the bone comprises a surface structure. In particular, with a spherical cap-shaped opening, into which a spherical head of the bone screw is inserted, the nearly vertical i.e. less inclined area of the opening facing away from the bone is provided with the surface structure which renders the support of the screw head more secure than in the inclined area. In that area which is oriented essentially vertically to the screw axis, the screw head moves, during screwing-in of the screw, essentially parallel to and along the inner surface of the opening. Only near the end of the screwing-in process is the lower area of the screw head supported on the inclined section of the spherical cap-shaped opening to thereby retain the bone plate on the bone.
In a preferred manner, the surface structure extends in the circumferential direction to achieve blockage in the circumferential direction, i.e. in the turning direction of the screw.
Preferred embodiments provide that the surface structure is shaped as longitudinal grooves, serration, fluting or the like. It is also feasible to produce the surface structure by surface roughening.
In a preferred embodiment, the longitudinal grooves or the serrations are formed as saw-tooth serrations. Therein, each saw-tooth of the serration has a steep and a flat flank. To achieve a blocking effect in the screwing-in direction of the screw, the flat flank rises in the screwing-in direction of the bone screw. Screwing-in of the bone screw is thus relatively easy and the steep flank of a saw-tooth of the serration prevents unscrewing thereof.
The blocking effect can be optimized in that the screw head is provided with a surface structure which supports the blocking effect. The screw head can be provided in particular with grooves or the like extending in the longitudinal direction. Saw-tooth serration is also feasible.
Further advantages, features and details of the invention can be extracted from the following description which describes in detail a particularly preferred embodiment with reference to the drawing. The features shown in the drawing and mentioned in the claims and in the description can thereby be essential to the invention either individually or collectively in any arbitrary combination.